Wheelchairs, especially electrically driven wheelchairs often have means for adjusting the position of the seat in relation to the chassis. These means may comprise a seat lift for vertically adjusting the height of the seat and a backrest tilt arrangement for tilting the backrest of the seat in relation to the bottom of the seat, i.e. for changing the angle between the backrest and the bottom of the seat.
Modern wheelchairs may additionally be provided with an electrically driven seat tilt apparatus, by which the entire seat, including the bottom and backrest may be tilted in relation to the chassis of the wheelchair. Such seat tilt apparatuses are provided for facilitating changing the body weight of the wheelchair occupant, to thereby avoid injuries, numbness and other inconveniences otherwise caused by static positioning of the occupant.
At one known type of seat tilt apparatuses the entire seat is tilted around a transverse horizontal axis, which axis is stationary in relation to the chassis. While allowing easy change of the occupant's body weight, this known type of seat tilt causes the center of gravity of the seat and occupant to move backwards during tilting. Such backward movement of the center of gravity is disadvantageous since it decreases the stability of the wheelchair and may even cause the wheelchair to fall over.
In order to solve this problem different so-called constant center of gravity seat tilt devices have been suggested. At these types of seat tilt arrangements the axis around which the seat is tilted moves forwardly during the tilting operation. U.S. Pat. No. 6,032,976 discloses one such apparatus, comprising a seat frame which, at a rear end, is pivotally connected to a transverse support member. The support member is guided in opposing horizontal linear slots arranged in a base frame of the wheelchair. The seat frame is also connected to the base frame by means of a tilt arm linkage, which is formed of two bars that are pivotally connected to opposing sides of the seat frame, forward of its rear end, and to the base frame. An electrical drive member is at a forward end connected to the base frame and at a rear end connected to the transverse support member. By actuating the drive member, the support member and the rear part of the seat frame is moved forwardly while being guided by the linear slots in the base member. During this movement, the two bars force the seat frame to tilt upwards around the pivot axis, which is defined by the opposing pivotal connections between the rear end of the seat frame and the transverse support member.
US 2007/0102615 A1 discloses another seat tilt apparatus comprising a base member with a guide slot, a seat supporting member which is movable in relation to the base member by means of a linear drive member and guided by the guide slot and a pivotal arm connecting the base member and the seat supporting member. For reducing the force required for initiating tilting and increasing the speed at which tilting is continued, the guide slot comprises two portions arranged at mutually different directions of extension.
Even though the arrangements according to U.S. Pat. No. 6,032,976 and US 2007/0102615 allow for tilting of the seat while keeping the center of gravity of the seat and occupant approximately constant in relation to the chassis of the wheelchair, these arrangements are impaired with certain disadvantages.